This invention relates to a method for refining steel.
Various prior art steel refining methods are well known and are disclosed in many texts, an example being "The Making, Shaping and Treating of Steel", 7th Edition, published by the U.S. Steel Corporation, copyrighted 1957, this text being hereby incorporated into this application for the benefit of any reader unfamiliar with such prior art methods.
A general characteristic of these prior art methods is that steel is both manufactured and refined in the same furnace from which it is tapped ready for casting. Therfore, the furnace must have a hearth which holds a volume of refined steel adequate for the casting requirement involved. This volume must be relatively large to meet normal steel plant requirements, particularly when it is to provide a feed for continuous steel casting equipment. In the case of duplexing, incompletely refined steel is produced by the converter, the open hearth or electric furnace which receives this steel then completing the refining in one hearth which must have adequate capacity for the casting requirements. Usually the refined melt is tapped into a teeming ladle from which the steel is teemed as required by the casting requirements, the steel thus being exposed to the atmosphere with undesirable results.
In the ASEA-SKF process, exemplified by the Swedish Pat. No. 323,156 and by the British Pat. No. 1,112,876, unrefined steel is poured in a ladle and while in this ladle is inductively stirred while covered by a pressure-tight cover having evacuating means for degassing, this cover is removed and replaced by a cover provided with arcing electrodes for the refinement, particularly for desulfurizing, possibly while also being inductively stirred, the ladle finally being uncovered and the steel teemed through a bottom teeming hole which is opened at that time. This practice requires a large investment in equipment and the application and removal of the different covers and other problems, although this practice has been commercially fully successful.
In general, all prior art steel making and refining practices have required the use of an extremely large investment in expensive equipment, in general the need to transfer the melt from one furnace to another or to a ladle, and in all cases have involved a substantial amount of atmospheric pollution and noise.
The Folgero and Fredrikson U.S. Pat. application Ser. No. 342,368, filed Mar. 19, 1973, now U.S. Pat. No. 3,851,090, granted Nov. 26, 1974 discloses and claims a new form of channel-type induction furnace particularly intended for use in melting down chips of metal and which is substantially free from atmospheric pollution and noise.
The above channel-type electric induction furnace has horizontally and vertically offset upper and lower gas-pressure-tightly covered hearths of substantial depths with an approximately vertical duct interconnecting the lower portions of the two hearths pressure-tightly. This duct has a normally closed tap hole in an upper portion, the two hearths having bottoms each provided with a channel-type inductor for heating and stirring metal melts in the hearths, the upper hearth's inductor being tilted towards the duct.
This furnace was invented for the purpose of melting down scrap metal, particularly brass chips, although the melting of all kinds of scrap metal was contemplated. The furnace construction is considerably less expensive than other types of electric furnaces capable of holding an equal volume of melt. The whole furnace is in the form of a closed unit; no loss of metal by oxidation and consequently a reduction in atmospheric pollution are inherent advantages.
Channel-type induction furnaces have not heretofore been used for the refinement of steel although they have been used in duplexing operations, particularly as a source of the melt for continuous casting operations. Such furnaces have been used to make steel additions, for deoxidizing, vacuum treatment and temperature adjustment. Channel-type induction furnaces have not been used in the steel industry for the refinement of steel for the following reasons:
The channel which must carry the melt loop forming the secondary of the inductor has not been able to withstand the high temperatures involved by steel refining operations in the case of a furnace having a hearth of adequate capacity to provide the volume of melt normally required for casting in the case of steel.
After casting of each melt, it has been necessary to leave a melt sump in the hearth and in the channel of the inductor. This is a particular disadvantage in the case of a large furnace if used for steel refining with the consequent requirement of the use of high temperatures with their destructive effects on the channel of a channel-type inductor furnace.